Start with grip, and understanding the notion of "grip it and rip it." You aren't "letting the disc go" as much as you're forcing the disc to "rip itself out." If you can't establish a firm grip that is consistently tight, absolutely nothing else anything anyone else tells you to do will result in any consistent throw that you can base your improvements off of.
Seek out grip tutorials and remember that the purpose of field work is to go outside of your comfort zone. Be happy when you grip a disc too tight in the field and yank the disc with a late release, and then remember how the release felt on that tight grip, and work on honing in an accurate release that generates that feeling in your hand. And if you are legit trying to practice gripping tightly as a new player, you WILL griplock, so don't get discouraged and start gripping softly.
My rule of thumb is that I tell players to over grip - grip the SHIT out of the disc, and then loosen up to merely really really firm/tight. Most new players don't go tight enough, and that seems to generally help them establish it.
As for your back issues - take the time to get loose. If you don't already do any yoga, I recommend it. Work on establishing a regular stretching routine to help you increase your rotational flexibility. And then when its time to play a round, spend a bit of time active-stretching, moving and working on slowly getting to your 'warmed up' level of rotational flexibility. As long as you're warmed up, given the legs should be doing most of the work, your back should be okay. Remember that the back is just in the middle of the process, its just a connecting piece in the middle of that whip (where your hand and the disc are the tip of the whip).
